Advantages of a cable-stayed bridge, which includes a main tower, a stay cable, and a girder as main components, are that it has good appearance and the stay cable serves as an elastic support for the girder and thus transmits a load of the girder to the main tower. Therefore, a bridge having a long span of 200 m or more as a length between main towers can be constructed in the form of a cable-stayed bridge, and such a cable-stayed bridge is mainly built over wide and deep river or sea.
Therefore, in constructing a cable-stayed bridge, distribution of force on a structure or a construction period is greatly affected by constructing methods of a stay cable and a girder, and finally, the economic feasibility of construction of a cable-stayed bridge depends on constructing methods of a stay cable and a girder. As a related-art cable-stayed bridge constructing method, a cantilever type constructing method, in which a girder is formed by installing segments on a main tower in sequence, is used. FIGS. 1 to 4 are schematic side views illustrating each of steps for constructing a cable-stayed bridge in a related-art cantilever type constructing method. In order to construct a cantilever type cable-stayed bridge according to the related-art constructing method, a main tower 100 is installed first and a segment 120, which consists of small blocks of about 10 m˜12 m, is situated in a bridge-axis direction (a longitudinal direction). A stay cable 110 is connected between the segment 120 and the main tower 100 and initial tension is introduced to the stay cable 110. In such a method, the segments 120 are installed in sequence and are connected to one another, so that a girder is formed. That is, as shown in FIGS. 1 to 4, the segments 120 are continuously installed in sequence from each of the main towers 100 using the stay cable 110 and the segments in the middle are bonded to one another, so that a girder connecting an entire span is formed.
However, since the related-art constructing method should perform the steps for installing the segment 120, anchoring the stay cable 120, and introducing the initial tension in sequent, it has a disadvantage of requiring much time to construct the entire bridge.
The initial tension introduced to the stay cable 110 when the segment 120 is installed is greater than tension exerted on the stay cable 110 by a load when the construction of the bridge is completed and the bridge is used. After the initial tension is introduced, the tension of the stay cable 110 is gradually decreased when the segments 120 are installed in sequence. In the related-art constructing method described above, since the initial tension greater than the tension in a practical use state should be introduced to the stay cable 110 in order to support the segment 120, tension greater than a load exerted in practice is exerted on the stay cable 110. To achieve this, the stay cable 110 should be manufactured bigger than that in the practice use state and thus unnecessary steel materials are consumed for the stay cable 110. Therefore, there are problems of a waste of resources and an increased cost.
Also, in the related-art constructing method, a great compressive force is generated on the segment 120 in a longitudinal direction (a bridge-axis direction) due to the initial tension introduced to the stay cable 110, and there is a disadvantage that a cross section of the segment 120 should be unnecessarily increased. Also, since the girder is constructed with the segment 120 having a longitudinal length of about 10 m˜12 m, a joint portion should be formed on every segment 120. Therefore, in order to connect the segments 120 one another, a plurality of connection plates are used and the number of processes of connecting the segments 120 such as high tension bolting or welding increase. Thus, there are problems of an increased cost and a delay in a construction period.
Also, in the related-art constructing method, since the steps for installing the segment 120, anchoring the stay cable, and introducing the initial tension should be performed in sequence, the segment 120 connected to one another forms a cantilever structure prior to completion of the bridge, and, such a cantilever structure of a long span is maintained for a long time during a bridge construction period. Therefore, this structure is vulnerable to a natural environment such as typhoon and an extra wind resisting means such as a stiffening cable to connect a lower portion of the segment 120 to the main tower 100 and support the segment 120 is required. Furthermore, since the tension exerted on each of the stay cables 110 is changeable during the bridge construction period, structural calculation to form a final bridge shape is complicated and much time and much money are required to design the bridge. That is, it is difficult to manage the shape, design, and construction of the girder.